hotrod deluxe IV v2b plate voltage

[aw73]

i was wondering if i lower v2b plate voltage with a larger plate load resistor r22 will the yellow and red more drive channel have a smoother less gritty and harsh overdrive? i seen someone suggest changing r75 to an 18 or 22k instead of the 10k but that would lower the headroom on the clean channel i think but maybe make it sound better? not sure.

[Stevem]

From my experience with messing around with changes like these and in terms of the plate or cathode resistor, you never want to change either more then 25% without changing the other.

This is the only way to allow the gain stage to amplify the full sign wave without clipping off the top or bottom of the sign wave well before the other clips

This form of distortion from clipping sounds like ass at least to many, and myself included.

If you do not have an Oscilloscope then now is the time to get cheap one.

Note this though.

When a sign wave gets amplified thru a gain stage it’s phased gets changed 180 degrees.

In other words for example what was the top half of the sign wave going in on the grid will come out being the bottom of the signwave from the plate.

Just clip out one end of that resistor and tack the one you want across it to audition that change.

You will not electronically harm anything by doing that and if you then like that change all around wise then leave it.

When you clip one end of the original 10k resistor clip it closer to the body so you end up the most of the lead sticking out of the board to solder to.


If you clip and solder carefully then you will not have to back the whole chassis out of that thing and disconnect a bunch of wires and un- nut the pots to do that.

[aw73]

From my experience with messing around with changes like these and in terms of the plate or cathode resistor, you never want to change either more then 25% without changing the other.

This is the only way to allow the gain stage to amplify the full sign wave without clipping off the top or bottom of the sign wave well before the other clips

This form of distortion from clipping sounds like ass at least to many, and myself included.

If you do not have an Oscilloscope then now is the time to get cheap one.

Note this though.

When a sign wave gets amplified thru a gain stage it’s phased gets changed 180 degrees.

In other words for example what was the top half of the sign wave going in on the grid will come out being the bottom of the signwave from the plate.

Just clip out one end of that resistor and tack the one you want across it to audition that change.

You will not electronically harm anything by doing that and if you then like that change all around wise then leave it.

When you clip one end of the original 10k resistor clip it closer to the body so you end up the most of the lead sticking out of the board to solder to.


If you clip and solder carefully then you will not have to back the whole chassis out of that thing and disconnect a bunch of wires and un- nut the pots to do that.

ok thanks. but isnt that where distortion comes from clipping the top and bottom of the wave or half of the wave? the overdrive/distortion already sounds like crap especially with the master lower so i dont have to worry about making it sound like crap...lol. so your saying its better to go bigger on r22, (maybe 18k or 22k), to change the plate voltage on all the preamp plates instead of just v2b? even my bb preamp AT version pedal i prefer it on 9 volts instead of 18 volts. it sounds less harsh, softer clipping and more saturation. this is my basis for changing the plate voltage. i can hear some of the same harshness even on the clean channel when the volume is up. on the yellow or red channel the overdrive sounds better when the master is on at least 5 but i dont want to have to wear ear plugs playing...lol.

[Stevem]

I don’t have time in this post I am replying to you with the level of details I would like to , but I will try to get across an important point on clipping/ distortion.

A preamp tube like in your amp and many others is biased in class A, it has to be biased in class to some level or another.

Circuit wise a tube does not need a cathode resistor to amplify, the problem with that is that the amplification you get out will only be 1/2 of the sign wave, in other words the positive half of the sign wave.

The tube must be biased up higher then just an idle level to amplify the negative half of the sign wave.

Now let’s say we now have a cathode resistor in place.

As your input signal gets larger the positive half will start to clip at the output and once the clipping reaches maximum that’s called the saturation point .
This is where any additional input signal makes no difference in the positive half of the sign wave.

And like I posted before this 1/2 of the sign wave is at the bottom .


Now in terms of the amplification of the negative 1/2 of the wave pushing the input signal level higher when you get clipping that’s called cut off.

Once it’s starts to clip it’s off just like a light bulb when you turn it off, there is no saturation just a nasty form of distortion.

If math wise you find your gain stage your working with needs for example to generate 3ma of clean positive signal and 3 ma of clean negative signal the tube then needs to be biased so that both haves of the sign wave clip equally .
Please someone clean up the bit of a mess I just made by trying to get this explanation posted in 5 minutes.

[pdf64]

Schematic?

[aw73]

I don’t have time in this post I am replying to you with the level of details I would like to , but I will try to get across an important point on clipping/ distortion.

A preamp tube like in your amp and many others is biased in class A, it has to be biased in class to some level or another.

Circuit wise a tube does not need a cathode resistor to amplify, the problem with that is that the amplification you get out will only be 1/2 of the sign wave, in other words the positive half of the sign wave.

The tube must be biased up higher then just an idle level to amplify the negative half of the sign wave.

Now let’s say we now have a cathode resistor in place.

As your input signal gets larger the positive half will start to clip at the output and once the clipping reaches maximum that’s called the saturation point .
This is where any additional input signal makes no difference in the positive half of the sign wave.

And like I posted before this 1/2 of the sign wave is at the bottom .


Now in terms of the amplification of the negative 1/2 of the wave pushing the input signal level higher when you get clipping that’s called cut off.

Once it’s starts to clip it’s off just like a light bulb when you turn it off, there is no saturation just a nasty form of distortion.

If math wise you find your gain stage your working with needs for example to generate 3ma of clean positive signal and 3 ma of clean negative signal the tube then needs to be biased so that both haves of the sign wave clip equally .
Please someone clean up the bit of a mess I just made by trying to get this explanation posted in 5 minutes.

ok thanks for taking the time to respond i appreciate any info. the reason i want to lower plate voltage is the same reason eddie van halen used a light dimmer to reduce 120v to something lower to get more saturation at a lower volume which the term brown sound came from. im thinking the overdrive will sound better with a lower plate voltage but im not sure. the schematic shows 345v at the rails before the plate resistors. with the master lower than 5 the overdrive gets worse as you turn down the master with more unwanted gritty/harsher overdrive. not sure why.

[aw73]

Schematic?

im not sure if its ok to post the IV schematic here. but i guess its ok to post the III schematic?

[aw73]

Schematic?

https://www.thetubestore.com/lib/thetub ... -Rev-B.pdf

[pdf64]

The HRDIII schematic is in the public domain, I can find that. Is it the same as the IV in regards of your queries?
If not, maybe post it here, a user needs to be logged in to see attachments.

[aw73]

The HRDIII schematic is in the public domain, I can find that. Is it the same as the IV in regards of your queries?
If not, maybe post it here, a user needs to be logged in to see attachments.

voltage is the same. c8 is 4.7, r24 is 1.5k, c3 and r27 on v2a gone, c23 is .001, r42 is gone, r20 is 220k and r19 is 100k, there is a resistor and cap (r27 39k and c3 .0015) from grid pin 7 on v2b going to ground. thats all i see thats changed for now.

also r18 change to 150k, c11 is 390pf, r42 is now 27k right after r18 going to ground.

[aw73]

Schematic?

i sent you a pm

[pdf64]

... with the master lower than 5 the overdrive gets worse as you turn down the master with more unwanted gritty/harsher overdrive. not sure why.

It might be parasitic capacitance between the clipping stage and later stages, after the master volume, allowing high frequencies to bleed / leak through.
At low settings, the leakage signal will be significant in comparison to the normal intended signal. Whereas a higher master settings, it will be relatively smaller.

Does setting the master volume to 5, and inserting a volume control into the fx loop to set the actual volume, help with the issue?

If not, increasing the value of C11 would help to remove high harmonics at source. eg to 1nF, 2n2F

[aw73]

... with the master lower than 5 the overdrive gets worse as you turn down the master with more unwanted gritty/harsher overdrive. not sure why.

It might be parasitic capacitance between the clipping stage and later stages, after the master volume, allowing high frequencies to bleed / leak through.
At low settings, the leakage signal will be significant in comparison to the normal intended signal. Whereas a higher master settings, it will be relatively smaller.

Does setting the master volume to 5, and inserting a volume control into the fx loop to set the actual volume, help with the issue?

If not, increasing the value of C11 would help to remove high harmonics at source.

the master volume doesnt help in the fx loop. i removed c11 then resoldered it. it allowed to much high frequency fizz. with the master down you loose alot of the body of (lower mids maybe) the amp. it sounds thin and gritty with the master low and almost more overdrive. it seems the master volumes resistance is have an effect on the preamp maybe or maybe if the master was behind the pi it would sound better?

[pdf64]

the master volume doesnt help in the fx loop.

Just to be clear, have you already tried an additional master volume patched into the fx loop?

Certainly removing C11 will allow more high harmonics, probably making the issue worse.
My suggestion, if the above doesn't help, is to increase the value of C11, above its stock value of 390pF, eg to 1nF or 2n2F.

Does switching more drive off help?

[aw73]

the master volume doesnt help in the fx loop.

Just to be clear, have you already tried an additional master volume patched into the fx loop?

Certainly removing C11 will allow more high harmonics, probably making the issue worse.
My suggestion, if the above doesn't help, is to increase the value of C11, above its stock value of 390pF, eg to 1nF or 2n2F.

Does switching more drive off help?

yes i put an additional master in the effects loop and turned the master on the amp all the way up. which i didnt think it would change anything since its next to it before the reverb. most amps i believe put the master after the pi but not sure if that would make a difference. turning the drive volume down helps but its still not great. i guess if you dont get the power tubes working then its not going to sound that good.

its the same on the yellow and red drive.